This application is based upon and claims priority from prior French Patent Application No. 0210747, filed Aug. 30, 2002, the disclosure of which is hereby incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to circuits generating sinusoidal frequencies, and more particularly to a converter circuit generating a sinusoidal signal.
2. Description of Related Art
The implementation of sinusoidal signal generator circuits is well known in the background art. In general, to generate a sinusoidal signal, one frequently starts from a periodic square signal, generated by a digital frequency synthesizer; this signal is then filtered through one or more band-pass filters in order to extract its fundamental harmonic. The advantage of such a process lies in the fact that the frequency of the generated sinusoidal signal can be set to the frequency of the square signal, thus benefiting from digital synthesizers"" precision. On the other hand, the large amount of odd harmonics contained in the periodic square signal makes filtering particularly difficult and it is then necessary to use more filters and to increase their complexity to reach a low distortion rate in the generated sine wave. Such complexity then results in a need for space on the semiconductor product, in an increased power consumption and finally in significantly high implementation costs.
However, in many circuits it is particularly desirable to benefit from the precision offered by a digital frequency synthesizer while preserving the purity of the generated sinusoidal signal. Then, the technical problem that must be solved consists in realizing a square-to-sinusoidal signal converter that is simple to implement and provides sufficient linearity for sophisticated applications, and in particular for frequency demodulator tuners. In addition, it would be desirable that the circuit could be easily incorporated into a semiconductor product.
Accordingly, a need exists to overcome the complexity of the prior art and to provide a sinusoidal signal generators circuit, which is easily to fabricate and works in a broad frequency range.
The present invention provides a sinusoidal signal generator circuit that is simple and inexpensive to implement.
Moreover, the present invention provides a square frequency converter, easy to incorporate into a semiconductor product and that makes it possible to generate a great quality sinusoidal signal in a broad frequency range.
The present invention includes of a sinusoidal frequency generator comprising an oscillating circuit based on the use of a gyrator. This gyrator consists of two transconductance amplifiers whose bias points are set by a control voltage (Vfilt) in order to regulate the oscillation frequency. The gyrator is easy to insert in a phase control loop in order to produce a square-to-sinusoidal frequency converter.
Accordingly, the present invention provides a sinusoidal generator that reduces the complexities of the prior art implementations and that provides benefits from the high quality coefficient of the gyrator, which is realized.
Preferably, each transconductance amplifier comprises:
a differential stage composed of a first transistor and a second transistor whose gates constitute the input of the stage;
a third transistor and a fourth transistor constituting active charges for the first and second transistors, respectively;
a fifth transistor mounted as a power source for the differential stage.
The bias current of the third, fourth and fifth transistors is controlled by control voltage (Vfilt).
In one embodiment, in, the bias point in each transconductance amplifier is controlled by a sixth transistor whose source is connected to a second reference voltage (Vdd) and whose drain receives control voltage (Vfilt). A seventh transistor (307) has a source connected to reference voltage (Vdd) and a gate connected to the gates of the sixth, third and fourth transistors. An eighth transistor, whose source is connected to ground, has a drain and a gate connected to the drain of the seventh transistor and to the gate of the fifth transistor that constitutes the power source for the differential stage. Thus, control voltage (Vfilt) controls the current in the sixth transistor, which current is then mirrored in the active charges composed of the third and fourth transistors, and in the power source formed by the fifth transistor.
Preferably, the signal generated by the gyrator is filtered by a filter having a structure similar to that of each of the amplifiers composing the gyrator, and whose bias point is also set by the control voltage (Vfilt).
Thus, it is possible to generate a sine wave having a total harmonic distortion (THD) higher than 35 dB, in a broad frequency range, for example between 70 and 130 MHz.
In other embodiments filters are added before the generator, to allow filtering of the generated sine wave, as needed.
The circuit is particularly adapted to the realization of a frequency tuner for satellite or cable reception.
The invention also allows realization of a converter for converting periodic square signal into sinusoidal signal, by a gyrator mounted as an oscillator and whose bias point is set by a control voltage Vfilt, the control voltage being generated by a phase control loop.